This invention relates generally to the monitoring and/or control of atmospheres within heat treating furnaces.
Steel parts can undergo a process called carburizing or neutral hardening inside a heat treating furnace. Inside the furnace, the steel parts are exposed to prescribed high temperature conditions in the presence of a specially formulated, carbon-enriched gas atmosphere.
Most heat treating atmospheres contain carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), hydrogen (H2), and water vapor (H2O). The relative amounts of these gases in the atmosphere depend upon the type of carrier gas used, the processing temperatures, and the amount of enriching gas added during processing.
For example, an endothermically generated gas, produced by catalytic cracking of natural gas in the presence of air, typically contains the following nominal ranges (expressed in % by volume)of gas constituents:
In gas carburization, a common commercial practice is to use an endothermic gas carrier enriched with natural gas or propane. The process variables used to monitor and control the carburization process using this type of atmosphere include (i) the carbon potential of the heat treating atmosphere (expressed as a weight percent of carbon), (ii) the temperature of the heat treating furnace, and (iii) the processing time.
For a given temperature condition, the reactions that transfer carbon to the surface of the steel part are maintained by keeping the carbon potential of the gas atmosphere within a defined range. For example, if the carbon potential of the furnace atmosphere is greater than the carbon potential of the surface of the steel parts being processed, carburization occurs, i.e., carbon is transferred from the gas atmosphere to the surface of the steel parts. Increasing the carbon potential of the gas atmosphere increases the rate of carburization. However, if the carbon potential of the atmosphere at a given temperature exceeds a critical value beyond the defined range, sooting occurs. Likewise, if the carbon potential of the atmosphere at a given temperature is less than the carbon potential of the surface of the steel parts being processed, decarburization occurs, i.e., carbon is transferred from the surface of the steel parts to the gas atmosphere.
The desired condition for neutral hardening is one in which the carbon potential of the atmosphere is equal to the carbon potential of the surface of the steel parts being processed. In this case, no carbon is transferred between the surface of the steel parts and the furnace atmosphere.
Further details regarding the concept of the carbon potential and the kinetic conditions for transfer of carbon between the surface of the steel part and the furnace atmosphere are found in Blumenthal, xe2x80x9cControl of Endothermic Generatorsxe2x80x94A Technical Comparison of Endothermic and Nitrogen/Methanol Carrier Atmosphere,xe2x80x9d Heat Treating Proceedings (16th ASM Heat Treating Society Conference and Exposition), Mar. 19-21, 1998, pp. 19 to 25.
The carbon potential of an atmosphere with a fixed carbon monoxide concentration can be ascertained by measuring the partial pressure of carbon dioxide (PCO2) in the atmosphere, using infrared analysis. This, however, requires sampling the gas from the furnace atmosphere and cooling it to room temperature. Sampling errors arise, due to possible leaks in the gas sampling line, alteration of the gas chemistry due to sooting, or the water-gas shift due to cooling, or a combination of these events. These sampling errors inherent in remote gas sampling are difficult to eliminate.
For this reason, a more common method for assessing the carbon potential has entailed the use of an in situ oxygen sensor used in association with a thermocouple.
The oxygen sensor is typically installed in the heat treating furnace in direct contact with the heated gas carburizing atmosphere. This obviates the sampling errors, described above, which are inherent in remote gas sampling techniques. The sensor includes a solid electrolyte. One side of the electrolyte contacts the carburizing atmosphere to be measured. The other side of the electrolyte contacts a reference gas, whose oxygen content is known.
A voltage (measured in millivolts) E(mv) is generated between the two sides of the electrolyte. The magnitude of this voltage E (mv) is a function of the temperature (sensed by the thermocouple) and the difference between the oxygen content in the carburizing atmosphere and the oxygen content in the reference gas. The voltage E(mv) can be expressed as follows:                               E          ⁡                      (            mv            )                          =                  0.0496          ⁢          T          xc3x97          log          ⁢                      xe2x80x83                    ⁢                                                    P                                  O                  ⁢                                      xe2x80x83                                    ⁢                  2                                            ⁡                              (                Ref                )                                                    P                              O                ⁢                                  xe2x80x83                                ⁢                2                                                                        (        1        )            
where:
T is the sensed temperature (in degrees Kelvin xc2x0 K).
PO2 (Ref) is the known partial pressure of oxygen in the reference gas, which in the illustrated embodiment is air at 0.209 atm. Other reference gases can be used.
PO2 is the partial pressure of oxygen in the heat treating atmosphere.
Knowing the oxygen content of the reference gas [PO2 (Ref)], one can determine the oxygen content of the furnace atmosphere [PO2] by measuring the probe voltage [E(mv)] and the temperature T(xc2x0 K). Knowing the carbon monoxide content of the carrier gas (which can be pre-set or separately measured by infrared analysis), the isothermal relationship between the oxygen probe voltage output and carbon potential can be experimentally ascertained and plotted for different temperature conditions. In this way, the carbon potential can be directly related to the oxygen probe voltage and temperature.
Further details of this relationship between oxygen probe voltage and carbon potential are found in the above-identified article by Blumenthal.
In use, a controller associated with the heat treating furnace compares the carbon potential of the furnace atmosphere to a xe2x80x9cset pointxe2x80x9d carbon potential, which is selected to reflect a targeted carbon potential. The controller controls the addition of an enriching gas, such as natural gas, into the atmosphere to maintain the carbon potential of the atmosphere at the set point, and thereby maintain the desired carbon potential in the atmosphere.
The control of carbon potential is only meaningful when the steel being processed is in a single phase field, i.e., austenite. This single phase field occurs only at elevated temperatures and is dependent on the alloy content, the carbon content of the steel, and the temperature.
There are other, lower temperature heat treating applications, e.g., spherodize annealing. In spherodize annealing, the objective is to create a two phase region, where the microstructure of the steel being processed comprises spherical-shaped particles of iron carbide(cementite)(Fe3C) distributed in a matrix of alpha iron (ferrite) (xcex1-Fe). This ferrite and iron carbide microstructure produces a steel that is very ductile and easily deformable by plastic deformation. The steel fastener industry, for example, depends upon steel that is in the spherodize annealed condition.
FIG. 1 shows a typical iron-carbon binary phase diagram for hypoeutectoid and hypereutectoid plain carbon steel compositions possessing different weight percentages of carbon. The diagram shows that there is a critical temperature A1 (about 1333xc2x0 F.) at which the desired two phase ferrite and iron carbide microstructure exists for both hypoeutectoid and hypereutectoid compositions. The most commercially practical spherodize annealing rates exist at or near the temperature A1. This preferred region is shaded in FIG. 1.
Below A1, the rate of spherodize annealing decreases with decreasing temperature for both hypereutectoid and hypoeutectoid compositions. At or below a temperature of about 1250xc2x0 F., the decreased rate becomes commercially impractical.
For hypoeutectoid compositions (i.e., with weight percent carbon below about 0.8), Above the temperature A1, equilibrium phases of austenite (xcex3-Fe) and ferrite occur, until the temperature exceeds A3. Above the temperature A3, single phase austenite forms, and spherodize annealing conditions cease. As FIG. 1 shows, the temperature corresponding to A3 varies according to the weight percent carbon of the hypoeutectoid composition.
For hypereutectoid compositions (i.e., having weight percent carbon above about 0.8), equilibrium phases of austenite and iron carbide form above the temperature A1, until the temperature reaches ACM. Above ACM, single phase austenite forms. As FIG. 1 shows, the temperature corresponding to ACM varies according to the weight percent carbon of the hypereutectoid composition.
During spherodize annealing, the objective is to provide a gas atmosphere that does not carburize or decarburize or oxidize the steel parts. The purpose of spherodize annealing is to produce the particular two-phase microstructure just described.
In the spherodize annealing process, the relative amounts of the two phases, e.g., ferrite and iron carbide, must remain unchanged. Thus, the heat treating atmosphere must be close to equilibrium with the two-phase ferrite and iron carbide. The atmosphere must be properly maintained to prevent either carburization (i.e., the conversion of ferrite with carbon from the atmosphere to produce iron carbide) or decarburization (i.e., the conversion of iron carbide to ferrite resulting from the removal of carbon by the atmosphere). The atmosphere must also prevent oxidation of the steel parts.
Carbon potential is meaningful only in a single phase region. Thus, the carbon potential is not an appropriate process variable for controlling the two-phase spherodize annealing process.
The activity of carbon (or AC) can be used as a process variable to control the furnace atmosphere to achieve the objectives of spherodize annealing. When the AC of a furnace atmosphere is equal to the AC of the two phase mixture of ferrite and iron carbide, the atmosphere is in equilibrium with the steel parts. In the equilibrium condition, there is no transfer of carbon between the steel and the furnace atmosphere.
AC is a function of the partial pressure of carbon monoxide (PCO) and the partial pressure of carbon dioxide (PCO) at a given temperature condition. AC can be expressed as follows:
AC=K1(PCO/PCO2)PCOxe2x80x83xe2x80x83(2)
where K1 is the thermodynamic constant of the reaction 2CO=C+CO2, which occurs during an exchange of carbon between the furnace atmosphere and the steel parts, where:
CO is carbon monoxide, CO2 is carbon dioxide, and C represents carbon in solution in iron.
AC may also be represented by the following equation:                               A          C                =                                            K              1                        100                    xc3x97                                                    (                                  %                  ⁢                                      xe2x80x83                                    ⁢                  CO                                )                            2                                      %              ⁢                              xe2x80x83                            ⁢                              CO                2                                                                        (        3        )            
where %CO is the percent carbon monoxide in the furnace atmosphere, and %CO2 is the percent of carbon dioxide in the furnace atmosphere.
There are atmospheres used for spherodize annealing of steel rod and wire, where relatively high concentrations of carbon monoxide are present, e.g., mixtures of exothermic and endothermic gases; mixtures of nitrogen and endothermic gases; and mixtures of nitrogen and methanol. These atmospheres are described in Stanescu, xe2x80x9cPrincipal Annealing Atmospheres for Steel Rod and Wire,xe2x80x9d Wire Journal(copyright) International, pp. 79-83, June 1991.
In these carbon monoxide-rich atmospheres, the %CO and %CO2 values in the atmosphere can be measured by removing atmosphere from the furnace for infrared analysis at room temperature. As already discussed, this technique is prone to many sampling errors. Nevertheless, in these carbon monoxide-rich atmospheres, the absolute value of the sampled ratio [(%CO)2/%CO2] can be at least quantified in some manner, albeit inexactly. The sampled ratio is proportional to AC, by virtue of the thermodynamic constant K1. The relationship between the sampled ratio and AC can be experimentally determined for a given set of operating conditions.
In use, a controller compares the measured ratio to a set point value. Based upon the comparison, the controller can govern the mixing of appropriate gases to keep the ratio at the designated set point value.
Nevertheless, the use of a sampled ratio [(%CO)2/%CO2] as an indication of AC is, at best, not exact and subject to a whole host of sampling errors. It also requires the presence of an atmosphere with a high concentration of carbon monoxide (i.e., xe2x80x9cricherxe2x80x9d), to enable the infrared analysis to be conducted in the first instance. These carbon monoxide-rich atmospheres are expensive to generate, highly kinetically reactive, and require the high degree of monitoring and control.
Atmospheres lower in carbon monoxide concentration (i.e., xe2x80x9cleanerxe2x80x9d) are well suited for use in applications where two phase regions exist, such as spherodize annealing. For example, as described in the above identified Stanescu article, carbon monoxide-lean atmospheres can be produced for spherodizing steel rod and wire by mixing nitrogen with a hydrocarbon (e.g., propane or propylene). The leaner atmospheres tend to be cheaper to generate, less kinetically reactive, and do not require the tight control that carbon monoxide-rich atmospheres require. However, due to the relatively low concentrations of carbon dioxide present in leaner atmospheres, the measurement of %CO2 by infrared analysis becomes, at best, problematic.
For example, at 1333xc2x0 F., K1=0.457, and if %CO=1% and AC=1, then, according to Equation (3), %CO20.0046%. The highest accuracy of a conventional CO2 analyzer is xc2x10.002%. It can thus be seen that, in these carbon monoxide-lean atmospheres, the accurate assessment of the activity of carbon from infrared measurements of %CO and %CO2 is not possible. Therefore, when a nitrogen-hydrocarbon atmosphere is used and the percent carbon monoxide is less than one percent, there exists no commercially viable way to accurately assess the activity of carbon during a spherodize annealing process.
In summary, to acquire even an error-prone sample ratio [(%CO)2/%CO2] requires the presence of an expensive, highly reactive, carbon monoxide-rich atmosphere (e.g., a nitrogen/endothermic atmosphere). In cheaper, less reactive, carbon monoxide-lean atmospheres (e.g., a nitrogen-hydrocarbon atmosphere), it is possible to acquire only a rough estimation of the activity of carbon for control purposes.
One aspect of the invention provides systems and methods for monitoring the activity of carbon in a heat treating atmosphere, e.g., where a two phase region is desired for spherodize annealing. The systems and methods generate a computed activity of carbon value for the gas atmosphere from at least one sensor placed in situ in the gas atmosphere.
Another aspect of the invention provides systems and methods for generating as a function of temperature, partial pressure of oxygen, and carbon monoxide content of the gas atmosphere, and without determining a carbon dioxide content of the gas atmosphere.
The systems and methods can make use of the computed activity of carbon value, e.g., to control the gas atmosphere.
In a preferred embodiment, the systems and methods compare the computed activity of carbon output to a set point activity of carbon value and generate a control signal based upon the comparison. In a preferred embodiment, the selected activity of carbon value is approximately one.
The invention makes possible the realization of accurate and reliable assessment of the activity of carbon in any two phase region, regardless of the carbon monoxide concentration of the atmosphere and without requiring a remote and error-prone analysis of the percent carbon dioxide.
Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended Claims.